Is the large investement into a CPU subzero cooling worth it? Given the fact that the unit can be easily refilled and the header can be changed to fit different types of old/new/yet to be released CPU sockets.

Subzero cooling increases your CPU speed between 50-100% depending on your system components. You can have next generation CPU speeds NOW with this cooling.

Some examples of commercially available Subzero coolings:
[g]asetek+VapoCHill+XE[/g]
[g]asetek+VapoCHill+LS[/g]

no longer available but older units can still be bought in select stores:
[g]nventiv+Mach+I[/g]
[g]nventiv+Mach+II[/g]

50% gain is also a bit of an overstatement if you compare the gains you can get from el cheapo aircooling or not so cheap but still much cheaper watercooling.

and sure, you can use it again when you upgrade your cpu but you'd have to go through quite a few cpu upgrades (those 1000€ cpu's not kept out of the equation) to justify the price of such a phase change unit.

It sucks, that's all.... It uses a Peltier element, meaning you need to power that too. Power = heat. Thus, in fact, you'll then have to cool down the heat of the CPU + the heat of the Peltier element.